The present invention relates to a cathode-ray tube and, more particularly, to a cathode-ray tube used in a display for a computer terminal or the like.
A phosphor mixture prepared by mixing a plurality of different color-emitting phosphors is used for a white phosphor screen in a display such as a computer terminal. Typical examples of the conventional phosphor mixture are listed below:
(1) a phosphor mixture of a blue-emitting silver-activating cubic zinc sulfate (ZnS:Ag) phosphor and a yellow-emitting (as a complementary color of blue) copper-activated zinc-cadmium sulfate {(Zn,Cd)S:Cu};
(2) a phosphor mixture of the above-mentioned blue-emitting phosphor (ZnS:Ag), a green-emitting manganese-arsenic-activated zinc silicate phosphor (Zn.sub.2 SiO.sub.4 :Mn,As), and a red-emitting manganese-activated zinc phosphate phosphor {Zn.sub.3 (PO.sub.4).sub.2 :Mn}or a manganese-activated zinc magnesium phosphate phosphor {(Zn,Mg).sub.3 (PO.sub.4).sub.2 :Mn};
(3) a phosphor mixture (Japanese Patent Disclosure (Kokai) No. 84-122578) of the above-mentioned blue-emitting phosphor (ZnS:Ag), the above-mentioned green-emitting phosphor (Zn.sub.2 SiO.sub.4 :Mn,As), and an orange-emitting cadmium halophosphate phosphor {Cd.sub.5 (PO.sub.4).sub.3 Cl:Mn}; and
(4) a phosphor mixture (Japanese Patent Disclosure (Kokai) No. 85-38490) of the above-mentioned blue-emitting phosphor (ZnS:Ag) and a yellow-emitting phosphor In.sub.l-u-v-w M.sub.u Tb.sub.v Eu.sub.w BO.sub.3 (wherein M is at least one element of Sc, Lu, Y, Gd, and Ga and u, v, and w satisfy u.gtoreq.0, v.gtoreq.0, w&gt;0, and u+v+w&lt;1).
Since each of the phosphors of phosphor mixture (1) has a 10% afterglow time of 10 msec or less, flickering of the cathode-ray tube typically occurs.
Since, in phosphor mixture (2), the 10% afterglow time of each of the green-emitting (Zn.sub.2 SiO.sub.4 :Mn,As) and red-emitting {Zn.sub.3 (PO.sub.4).sub.2 :Mn}and {(Zn,Mg).sub.3 (PO.sub.4)2:Mn}is relatively long, flickering of the screen is reduced. However, phosphor mixture (2) contains the red-emitting phosphor having a low luminous efficacy, and thus, the brightness of the screen is degraded. In addition, phosphor mixture (2) contains arsenic, which is highly toxic, and is thus undesirable from the viewpoint of safety.
In phosphor mixture (3), brightness and flickering characteristics are improved as compared with phosphor mixture (2). However, mixture (3) contains arsenic and cadmium which are highly toxic, thus posing a safety problem.
Phosphor mixture (4) is free from toxicity and has a high luminance. However, the afterglow time of the In.sub.l-u-v-w M.sub.u Tb.sub.v Eu.sub.w BO.sub.3 phosphor is shorter than that of each of the aforementioned Zn.sub.2 SiO.sub.4 :Mn,As, Zn.sub.3 (PO.sub.4).sub.2 :Mn, (Zn,Mg).sub.3 (PO.sub.4).sub.2 :Mn, and Cd.sub.5 (PO.sub.4).sub.3 Cl:Mn phosphors. Therefore, flickering characteristics of phosphor mixture (4) are degraded.
In order to improve the flickering characteristics of phosphor mixture (4), the present inventors replaced the cubic zinc sulfate phosphor as the blue-emitting phosphor with a hexagonal zinc sulfate phosphor (ZnS:Ag) disclosed in Japanese Patent Disclosure Nos. 83-115024 and 83-129083 and the like. As a result, the flickering characteristics could be improved.
However, the hexagonal zinc sulfate phosphor has poor current dependence of brightness. If the screen is made bright by increasing an exciting current in a cathode-ray tube using the above phosphor mixture, white color cannot be satisfactorily saturated and becomes yellowish.